1. Field of the Invention
The present invention relates to water-absorbing composites containing water-absorbing polymer particles in the form of agglomerates immobilized on a fibrous substrate and preparation processes thereof. More specifically, the present invention relates to water-absorbing composites containing agglomerates of water-absorbing polymer particles stably immobilized on a fibrous substrate and having good water-absorbing properties, a high water-absorbing speed and an excellent immobility of swollen gel after absorbing water as well as preparation processes thereof. The present invention also relates to water-absorbing articles using a water-absorbing composite having these characteristics.
2. Description of the Background
Paper, pulp, nonwoven fabric, sponge-like urethane resins or the like have been used as water-retaining materials in various sanitary goods such as sanitary napkins, paper diapers and agricultural materials, for example. However, the water-absorbing capacity of these water-absorbing materials is only 10-50 times their own weight. Thus, larger amounts of materials are required to absorb or retain larger amounts of water, consequently leading to enormous bulk. These materials also have the disadvantage that they readily release water when pressurized after absorbing water.
For the purpose of lessening such disadvantages of these types of materials, various highly water-absorbing polymer materials have recently been proposed, such as starch graft polymers (JP-B 46199/88), modified cellulose (JP-A 80376/75), crosslinked water-soluble polymers (JP-B 23462/68), self-crosslinked acrylic alkali metal salt polymers (JP-B 30710/79), etc.
These water-absorbing polymers exhibit high level of water-absorbing performance, but most are powdery. Accordingly, such water-absorbing polymer powders must be uniformly dispersed on a substrate such as tissue, nonwoven fabric or paper for use as sanitary materials such as sanitary napkins or paper diapers. However, water-absorbing polymer powders dispersed by known methods have the following disadvantages. First, they are difficult to stably immobilize on a fibrous substrate, and even if uniformly dispersed, they often become locally concentrated. Second, swollen gels after absorbing water fail to stably remain on a fibrous substrate but readily move from the fibrous substrate.
When such polymer powders as noted above are uniformly dispersed on a fibrous substrate to give an absorbent, not only are the polymer powders readily separated from the fibrous substrate, but very high cost occurs due to complexity of powder handling and process design for efficient uniform dispersion.
Conventional approaches to addressing these problems include, for example, fixing polymer powder on a fibrous substrate with a binder or coating an aqueous polyacrylic metal salt solution on a substrate and then introducing crosslinkage by a heat-drying process.
However, the former suffers from complexity due to the use of the binder and the latter also has drawbacks, such as insufficient water-absorbing performance.
A process for preparing a water-absorbing composite by preparing a composite containing an aqueous acrylic monomer solution applied in a predetermined pattern on a shaped fibrous substrate and irradiating it with electromagnetic radiations or particulate ionizing radiations to convert the acrylic monomer into a water-swelling polymer has been reported (JP-B 67712/91). This process affords an improved powder handling, such as formation of a uniform dispersion and stable immobilization on a fibrous substrate as described above, but it has the disadvantage that the electromagnetic radiation or particulate ionizing radiation used to convert the acrylic monomer into a water-absorbing polymer markedly accelerates self crosslinking reaction of the water-absorbing polymer to greatly lessen the performance as a water-absorbing material, especially water-absorbing capacity, i.e. usually to half of that obtained with the highly water-absorbing polymer powders previously described. In particular, the highly polymerizable aqueous acrylic monomer solution is absorbed into the fibrous substrate itself to form a very hard plate-like composite after polymerization, which must be broken for actual use. Moreover, absorbing capacity, especially water-absorbing capacity is greatly lowered by the inhibitory effect of the fibrous substrate against swelling of the water-absorbing polymer.
An absorbing article containing a fibrous substrate and a water-absorbing polymer wherein a part of the water-absorbing polymer nearly spherically encloses and discontinuously adheres to the substrate has also been proposed (JP-B 58030/93). This absorbing article affords partially improved water-absorbing characteristics over the above prior products, but it has the disadvantages that hydrophilic fibers are unsuitable for the fibrous substrate though this is a water-absorbing article, and that the water-absorbing polymer loses adhesion and tends to be readily separated after becoming swollen with absorbed water.
Hence, a need exists for a water-absorbing composite which overcomes the above disadvantages.
Quite surprisingly, it has now been discovered that a water-absorbing composite with excellent water-absorbing properties and water-absorbing speed wherein highly water-absorbing polymer particles are stably immobilized on a fibrous substrate can be prepared by polymerizing droplets of a mixed solution of an aqueous polymerizable acrylic monomer solution initialized with a redox polymerization initiator on a fibrous substrate (JP-A 67403/97).
Accordingly, it is an object of the present invention is to provide a water-absorbing composite with excellent water-absorbing properties and a high water-absorbing speed wherein most of the highly water-absorbing polymer is stably immobilized on a fibrous substrate and the immobility of swollen gel after absorbing water is also excellent and a process for preparing it.
It is also an object of the present invention to provide a water-absorbing article with high practicability using a water-absorbing composite having said properties.
The above objects and others are provided, in part, by a water-absorbing composite, containing water-absorbing polymer particles immobilized on a fibrous substrate wherein at least part of the water-absorbing polymer particles contain primary particles having an average particle diameter of about 50-1,000 xcexcm, wherein about 30% by weight or more of the primary particles are considered to form agglomerates having a shape satisfying the following conditions while nearly maintaining their primary particle shapes and a part of particles of the agglomerates are not adhered to the fibrous substrate:
Average particle diameter (D) 100xe2x89xa6Dxe2x89xa63000 xcexcm
Average relative displacement of the direction by direction analysis (xcex8) 10xe2x89xa6xcex8xe2x89xa625
Frequency analysis 5 Hz/20 Hz intensity ratio (k) 0.6xe2x89xa6kxe2x89xa60.9
Agglomerate maximum length (L)/minimum length (l) ratio 1.2xe2x89xa6L/lxe2x89xa615.0.